Sensing viruses: shape matters

Viruses are basically packets of nucleic acid, DNA or it’s sister molecule RNA. Our cells have therefore evolved to recognise these molecules as a sign of virus infection. A recent study from Jan Rehwinkel’s lab in the MRC Human Immunology Unit has revealed a new way in which cells sense and respond to invading viruses. Layal Liverpool, a DPhil student in the Rehwinkel lab, who was involved in the work, explains more.

Infection is one of the biggest threats faced by our cells. To combat this, cells have evolved a highly-tuned detection system that relies on proteins called sensors. Viruses rely on nucleic acids, DNA or RNA, to infect cells and propagate themselves. Nucleic acid sensors in our cells exploit this by recognising DNA or RNA from invading viruses and activating potent anti-viral responses.

These early anti-viral responses are the cell’s first line of defence against an invading virus. One such response is a form of cell suicide called necroptosis. One of the nucleic acid sensors, called ZBP1 or DAI, can activate this form of cell death upon detecting herpes virus infection. By committing suicide, the infected cell sacrifices itself to stop the spread of the virus to other cells in the body.

Necroptosis is a “messy” form of cell death because it essentially involves the explosion of the cell, releasing all its contents into its surroundings. Here they can be picked up by patrolling immune cells, alerting them to the fact that there is a virus around.

Although ZBP1 was already known to activate cell suicide, exactly what from the virus ZBP1 was detecting had been a mystery. In their study, Jonathan Maelfait and colleagues in Jan Rehwinkel’s lab demonstrate that ZBP1 recognises RNA from the invading virus.

Like viruses, our cells also contain their own nucleic acids, for example the DNA that makes up our genes. A big question then is how nucleic acid sensors, like ZBP1, can tell the difference between the cells own nucleic acids and those coming from invading viruses.

Well, it might all come down to shape. In addition to the classical DNA double helix structure on the cover of every GCSE biology textbook, DNA – and RNA – can adopt another, more jagged, zig-zag shape named ‘Z’. In fact, ZBP1 is so-called because of its special ability to recognise Z-shape nucleic acids.

Maelfait and colleagues found that when they made cells with a broken version of ZBP1, which could no longer recognise Z-nucleic acids, the cells lost the ability to activate cell suicide in response to virus infection. Mice that have the same broken version of ZBP1 are more vulnerable to virus infection, probably because the lack of cell suicide allows the virus to persist and spread through the body more easily. This discovery suggests that the Z-shape might be an important molecular sign of virus infection, allowing cells to distinguish their own nucleic acids from those of viruses.

Understanding exactly how our cells make this distinction is important because when it goes wrong, it can lead to auto-immune or auto-inflammatory diseases. Clearly it would not be useful for our cells to start committing suicide when there is no virus! In the future, further investigation of nucleic acid sensing may reveal novel therapeutic targets for the treatment of viral infections and auto-inflammatory diseases.